Hot melt adhesive systems have many applications in manufacturing and packaging. For example, thermoplastic hot melt adhesive materials are used for carton sealing, case sealing, tray forming, pallet stabilization, nonwoven applications including diaper manufacturing, and many other applications. Typically, hot melt adhesive materials are contained in or provided from an adhesive supply, such as a tank or hopper of an adhesive melter. The hot melt adhesive material is heated, melted, and pumped to a dispenser, such as a dispensing gun or other applicator which applies the hot melt adhesive material to a carton, case, or other objects or substrates. For the adhesive supply, different types of melters have been developed, including tank-style melters and grid and reservoir melters. In a tank-style melter, heating elements increase the temperature of one or more surfaces of the tank and the hot melt adhesive materials inside the tank. In a grid and reservoir melter, hot melt adhesive material moving through a tank or hopper heats on a grid of heating elements and melts as it passes from the grid into a reservoir, which is also heated. Manifolds are used to direct liquid hot melt adhesive material into plural flow streams for output through hoses to dispensers. Heaters are typically thermally connected to several components of a hot melt adhesive system, including the adhesive supply (such as a tank, grid, reservoir), manifold, hoses, and dispenser. The heaters maintain the hot melt adhesive material at proper adhesive viscosity and temperature.
In addition, different types of pumps have been developed for use in hot melt adhesive systems. Piston pumps, for example, use a piston to move a hydraulic plunger, which drives liquid hot melt adhesive material through the hot melt adhesive system. And gear pumps employ counter-rotating gears to create positive displacement for precise metering of liquid hot melt adhesive material. Pumps move the liquid hot melt adhesive material through the hot melt adhesive system, including through the hoses and to the dispenser for application to an object.
Metering systems have been developed to monitor or control the flow of hot melt adhesive material in hot melt adhesive dispensing systems. In a typical conventional metering system, for example, one or more material supply pumps are used to pump hot melt adhesive material and deliver it to a point of application. For example, one material supply pump, generally positioned at or near a supply of hot melt adhesive material (such as near a reservoir containing hot melt adhesive material) and referred to as a sump pump, provides pressurized hot melt adhesive material to a downstream metering station, which is sometimes referred to in the art as a remote metering station. In some cases, a manifold is included with a remote metering station to split a flow stream of hot melt adhesive material into plural flow streams for output through hoses to dispensers. Another material supply pump may be included with the remote metering station, or between the sump pump and the remote metering station, to assist in pumping the hot melt adhesive material. In such an arrangement having two pumps, the sump pump may be associated with a closed-loop feedback system, whereas the downstream material supply pump may be associated with an open-loop feedback system.
Generally, a closed-loop feedback system is a process control technique that uses feedback of a process output to adjust the variables impacting the output. An open-loop feedback system, in contrast, is a process control technique that does not use output feedback. For example, in the conventional metering system described above, the sump pump may be associated with a downstream flow sensor or meter to measure flow from the sump pump. The measured flow is considered by a controller associated with the sump pump, and the controller adjusts the sump pump in response to the measured flow. For example, the controller may increase or decrease the speed at which the sump pump operates in order to change the flow rate of hot melt adhesive material from the sump pump. The downstream material supply pump, may not necessarily be associated with a closed-loop feedback system, so the flow from it is not measured and the pump is not adjusted in response to a measured flow. By not measuring the output flow from the downstream material supply pump, information about the actual flow of hot melt adhesive material flowing therefrom toward the point of application is not collected.
Metering systems have also been developed that include downstream material supply pumps having closed-loop feedback systems. In such systems, an auxiliary controller and/or other auxiliary components are introduced to an adhesive dispensing system to provide the closed-loop feedback system for the downstream material supply pump. For example, an auxiliary sensor may be introduced with or downstream from the material supply pump for measuring the rate of hot melt adhesive material flow. This measurement is sent to the auxiliary controller. The auxiliary controller, in turn, communicates with other control components of the hot melt adhesive system that control the downstream material supply pump. These other control components, in turn, adjust the material supply pump in order that the measured flow more closely matches a target flow. When auxiliary controllers or components are added to a metering system, they have been housed in components separate from, and in addition to, the principal components of the hot melt adhesive dispensing system, including its metering system.
In other metering systems, only a single material supply pump is used, especially in circumstances where the supply of hot melt adhesive material is in close proximity to the point of application (less than 10 meters apart, for example).
There is a need, therefore, for a hot melt adhesive dispensing unit and metering systems for use therewith that address one or more of the shortcomings discussed above.